The present invention relates to an indicator control circuit that converts a measurement value of a physical quantity into digital data in order to perform display on an indicator such as a speed meter, a tachometer, a water temperature gauge and a fuel gauge, and that performs display on instruments in accordance with that digital data, and in particular, relates to an indicator control circuit that generates a hysteresis in a digital filter without the use of a hysteresis circuit.
A digital display indicator differs from analog display indicator in that the display of the speed of an automobile is given in digital values, for example.
One problem with an indicator that performs digital display is that even when an automobile is running at a constant speed, the value displayed is scattered around the value that is the indicated speed. On the other hand, it is necessary to have the prompt renewal of the displayed speed in accordance with changes in the speed when the vehicle is accelerating and decelerating but there is a poor following. One means of eliminating these problems is to provide a hysteresis circuit. However, although such hysteresis circuits enable the scattering of the display value of the instrument to be reduced, they have an overall configuration which is complex and also have a high cost.
FIG. 1 is a block diagram of one example of a conventional indicator control circuit. A signal conversion circuit 21 is configured from a waveform rectifier circuit 12 and a frequency count circuit 13, and the input signals to be measured which are input from an input terminal 10 are converted into digital data, and the converted digital data is supplied to a digital filter circuit 14 so that the response characteristics can be adjusted, and then drive signals corresponding to digital data are generated in the drive circuit 19 and supplied to the display device 20 and drive the gauge of the display device 20.
A digital filter circuit 14 can be configured most simply by a primary filter but they can also be configured from secondary and later filters. FIG. 2 indicates an example of a digital filter circuit 14 that has a complex filter configuration. This digital filter circuit 14 is configured from multipliers 34 to 44, adders 45 to 52, and a delay circuits 53 to 55.
As can be seen from this configuration, in a conventional indicator control circuit, there is no circuit to prevent the flickering of the display value on the instrument when there is a constant speed. Accordingly, there is the problem of flickering of the display value on the display indicator when a hysteresis circuit is not provided.
FIG. 3 indicates a display indicator that is provided with a hysteresis circuit. The flickering of the display value is reduced by a hysteresis circuit 23. The hysteresis circuit 23 is set so that the value displayed for the speed is renewed only when the difference between a speed displayed on a display portion 25 and a speed corresponding a count value in a vehicle speed counter 27 is greater than a predetermined value.
In addition, an indicator control circuit, shown in FIG. 3, also has zero display judgment circuit 29. The zero judgment circuit 29 outputs 0km/h data when the speed is less than a predetermined value, and then the Okm/h is displayed on the display portion.